1. Field of the Invention
This invention relates to a process for producing a toner to be used for a method for visualizing electrostatic latent images.
2. Related Background Art
Toner is a material for forming and recording images. For example, a large number of electrophotographic methods have been known as described in U.S. Pat. No. 2,297,691. Generally speaking, by utilizing a photoconductive material, an electrostatic latent image is formed by various means on a photosensitive material, then said latent image is developed by use of a toner, and after transfer of the toner image onto a transfer member such as paper if necessary, the developed image is fixed by heating, pressure, heating pressurization or a solvent vapor to obtain a copied product. Various methods for developing of the toner and fixing toner have been proposed in the prior art and used as desired.
In the prior art, as the toner to be used for these purposes, the toner having desired particle sizes is produced generally by fusion mixing magnetic powder, a colorant such as dye or pigment into a thermoplastic resin to have them uniformly dispersed therein, cooling the kneaded product, crushing the cooled product by means of a micropulverizing device and classifying the resultant powder by means of a classifying machine.
The production process according to the crushing method can produce a considerably excellent toner, but the material for toner is limited in the scope of selection. For example, the dispersion composition of the resin and the colorant must be one which is sufficiently brittle and can be finely pulverized economically effectively. From this demand, the dispersed composition of the resin and the colorant is made brittle, and therefore particles with wide scope of particle sizes are liable to be formed when micropulverized at high speed, whereby there ensues the problem that fine particles are included therein with a relatively large proportion. Further, such brittle dispersed composition, when used in development of latent images in a image forming device such as a copying machine, is further subjected to micropulverization or powdering by mechanical shock. According to this method, it is difficult to disperse completely solid fine particles such as magnetic powder or colorant into the resin. Depending on the degree of the dispersion, it may cause increase of fog or lowering in image density, and therefore care must be paid to dispersion. By exposure of an electroconductive material such as magnetic material on the broken surface, it may sometimes become difficult to produce an insulating toner.
On the other hand, in order to overcome the problems of the toner according to these crushing methods, a process for producing a toner according to the suspension polymerization method has been proposed. In the suspension polymerization method, a monomer composition comprising a polymerizable monomer, a polymerization initiator and further optionally a crosslinking agent, a charge controller, a magnetic material, a colorant and other additives dissolved or dispersed uniformly is dispersed into an aqueous phase containing a dispersing stabilizer by means of a stirrer and polymerization reaction is carried out to obtain toner particles having desired particle sizes.
The toner obtained by such polymerization reaction requires no crushing step, and therefore has substantially no problem based on crushing such as nonuniformness of the broken surfaces by crushing and nonuniformness of shape, and the shape is also spherical as preferable for the toner.
Nonuniformness between the toner particles according to the crushing method has great influences on flowability, triboelectric charging characteristic to affect developing characteristic of the toner, changes or deterioration of the durability performance.
Since the toner obtained by the above polymerization reaction forms toner particles through polymerization of the monomer droplets of the monomer composition substantially dispersed in the aqueous phase, the toner particles are influenced by the state of the monomer droplets. Thus, it is an important problem how homogeneous is made the state of monomer droplets. For example, it is important how to make homogeneous the dispersion of the colorant in the monomer droplets, the content of the colorant, dispersion of the additives, contents of the additives, dispersion of the polymerization initiator and the content of the polymerization initiator are important factors.
In the prior art, for solving these problems, the method for treatment of a colorant or the method for adding a dispersing agent for the colorant have been proposed, but these methods were not yet satisfactory.